PROJECT SUMMARY Combined anti-retroviral therapy (cART) and pre-exposure prophylaxis (PrEP) represent major milestones in the effort to eliminate AIDS and prevent new HIV-1 infections. They nonetheless have limitations. For example, a life-time use of two or three compounds delivered to most every cell and tissue in the body will likely come with undesirable, difficult-to-anticipate side effects. Access and compliance also remain concerns, especially among infected persons who have not yet been reached by our healthcare infrastructures. Similarly, PrEP requires both access and a conscious effort before a potential transmission event, something that is not always realistic for the hardest-to-reach demographics here and abroad. Here we will develop an approach that provides robust prophylaxis and perhaps effective viral suppression for six months or more after a single injection. Specifically we will optimize eCD4-Ig, a very broad and potent antibody-like molecule, for its delivery in an injectable hydrogel, and we will optimize this hydrogel for delivery of eCD4-Ig. eCD4-Ig provides highly effective protection in rhesus macaques from high-dose challenges with both SHIV-AD8 and SIVmac239. It also has the breadth and potency to suppress an established SHIV-AD8 infection. This breadth appears sufficient to suppress the wide diversity of viruses in an individual and in a population. As importantly, HIV-1 has not developed easily accessible pathways of escape from eCD4-Ig as it has from neutralizing antibodies. It is therefore an ideal payload for a safe, effective, and sustained hydrogel delivery system. As we show, these hydrogels are well-tolerated, non-immunogenic, easily manufactured, and deliverable with a high-gauge need. Importantly, they and their payloads can be immediately withdrawn in case of an adverse event. Modeling suggest that they can sustain eCD4-Ig concentrations that could provide effective prophylaxis for well over six months. We will test this possibility in human FcRn- transgenic mice and in rhesus macaques, and confirm that our best eCD4-Ig/hydrogel formulations could replace PrEP and/or cART.